(1) Study of anomalous submarine earthquakes by ocean acoustic waves An anomalous submarine earthquake with the CLVD mechanism happened near Torishima Island. We found that this earthquake generated a series of ocean acoustic wave trains. The analysis of the OBS records of these waves lead us to conclude that the crustal deformation associated with the CLVD earthquake squeezed magma fluid in the source region upward, that the ascent of the magma fluid generated volcanic earthquakes whose foci migrated upward progressively, and that these earthquakes generated a series of ocean acoustic wave trains.(2) Detection of submarine seismic activity by ocean-coupled shear waves We detected a series of events that generated "T phase-like" waves and found that they were the shallowest submarine seismic activity in a very localized area off Boso, Japan. Our theoretical study indicates that the observed "T-like" waves are the sediment-trapped shear waves coupled with ocean acoustic waves multiplly reflected at the sea surface. We named these waves "ocean-coupled sedimentary shear waves" which should be useful to detect any submarine shallowest activity.(3) Study of submarine volcanic activity by ocean acoustic waves The seismograms of OBS cable arrays often record ocean acoustic waves due to submarine volcanic activities. We located the origins of these waves near the northern end of the Mariana volcanic arc. The analysis of these waves indicate that the volcanic activity often starts gradually and stops abruptly and that ocean acoustic waves become more rich in lower-frequency content as the activity becomes more intense. We also detected a semi-diurnal migration of the T-phase origins, which is perhaps an apparent one due to the tidal effect of the velocity structure of the ocean.